Secondary-electron-emitting surface



Filed July-l, 1949 Z/ l 22 l ZZ lllnr vlll.||||||ll/ INVENTOR fam/.eff

Patented Dec. 2, 1,952

2,620,287 SECONDARY-ELECTRON-EMITTING RFACE Jenny Bramley, Long Branch, N. J.

Application July 1, 1949, Serial No. 102,618

7 Claims. l

My invention relates to processes of secondary-electron-emission.

The present application is a continuation in part of my application Serial No. 612,197, filed August 23, 1945, for Secondary-Electron-Emitting Surface, now Patent No. 2,548,514, granted April 10, 1951, including the species in which the dielectric is formed from the metallic base. Certain apparatus subject matter of that case is included in my application Serial No. 20,016, filed April 9, 1948, for Secondary-Electron-Emission, now Patent No. 2,527,981, granted October 31, 1950.

The primary purpose of this invention is to obtain multiplication of secondary electrons much higher than that obtained in prior practice. This is accomplished in this invention by the proper construction of composite surfaces. One method is to interpose a layer of dielectric limited in thickness to 0.1 mm. and preferably not exceeding 0.03 mm. between a layer with at least moderately high secondary-electron-emissive properties (called henceforth the secondaryelectron-emitting layer) on the one hand and a metallic base on the other hand. In accordance with the present invention the dielectric layer is a self-limiting oxide layer formed from a metal of the metallic base. For the secondary-electronemitting layer the following substances for example may be used: beryllium or an alloy or oxide thereof, magnesium (oxidized), aluminum, or an alloy or oxide thereof.

A further purpose relates to methods for increasing the emission of secondary electrons.

A further purpose is to obtain high multiplication in a secondary-electron-emitter without the undesirable features of photoelectric effect due to the presence of a photoelectric material, such as caesium, in the layer emitting secondary electrons.

A further purpose is to produce a secondaryelectron-emitter of high multiplication while avoiding time lag between the start or stop of the primary current and the start or stop of high secondary electron emission by providing for the neutralization of the positive charge from the secondary-electron-emitting layer at a rate determined by the primary electron beam. One way of accomplishing this is to separate the secondary-electron-emitting layer from the metal base by an extrinsic semi-conductor (N or P type) which can show metallic conduction properties under high electric elds, which can either be applied across the dielectric or generated by the bombarding electrons. An extrinsic semi-conductor is a material which at room temperature acts as ametallic conductor under high voltages but not under low voltages, and at higher temperatures Without changing state acts as a metallic conductor at both high and low voltages. See H. C. Torrey and C. A. Whitmer, Crystal Rectiers (McGraw-Hill) page 47.

A semi-conductor in which the conduction is solely by electrons at low temperatures is an N- type semi-conductor. A semi-conductor in which the conduction is solely by holes at low temperatures is a P-type semi-conductor (G. L. Pearson, The Physics of Electronic Semi-Conductors, 66 Transactions of AIEEI 209 (1947)).

A further purpose is to maintain close time coordination between a secondary-electron-emitter and the primary current by providing for the neutralization by electrons of the positive charge in the secondary-electron-emitting layer at a rate predetermined by the primary current.

A further purpose'is to enhance the secondaryelectron-emission of the secondary-electronemitting layer of a composite surface, such as magnesium oxide by baking it in vacuo at a temperature of 450 to 800 C., preferably 600 to 800 C.

A further purpose is to employ as a metallic base a metal such as aluminum, chromium, or copper, or an alloy thereof, which will form an adherent self-limiting oxide capable of acting as the dielectric,.and to apply a secondary-electron-emitting layer over the oxide.

A further purpose is to decrease conductivity along the layer of the secondary-electron-emitter in the composite surface as by granulating the surface into a mosaic.

A further purpose is to oxidize a metallic base, such as aluminum, copper or chromium, or an alloy thereof, or a corrosion resisting iron-chromium alloy, to form a dielectric, apply over it a secondary-electron-emitting layer and expose the layers to an elevated temperature for a short time to form a mosaic.

A further purpose is to use a metallic base of a metal forming an adherent oxide, such as aluminum, copper or chromium, or an alloy thereof, oxidize the surface of the base,.thus forming a dielectric layer over the base; coat the surface with a thin film of secondary-electron-emitter which may be deposited by any suitable method, such as dusting or settling for a ne particle material, evaporation or electrode-sputtering, or mechanical deposition ifv the material is obtainable in thin sheets. Y

A further purpose is to construct a secondaryelectron-emitter from an alloy of a metal forming an oxide of suitable dielectric properties, such as aluminum, and a metal which is a good secondary emitter, such as beryllium or magnesium, to bring the secondary-electron-emitter to the surface and to oxidize the surface of the alloy in such a way as to form a layer emitting secondary electrons over a dielectric layer. The alloy must be free from any poisoning impurities, such as nickel. A suitable method of bringing the secondaryelectron-emitter to the surface is hydrogen firing or burnishing under an inert liquid, such as b911- zene.

A further purpose is to reduce the electrical input in a tube containing a thermionic cathode by using a small cathode requiring only a small current to heat it and obtaining high secondaryelectron-emission from an auxiliary cathode, preferably in mesh form, constructed in accor-dance with this invention.

A further purpose is to employ an auxiliary cathode constructed in accordance with this invention to overcome prior art difiiculties, such as grid emission due to grid contamination from oxide-coated cathodes, backring, and ignition of local spots. Since the primary thermionic current is only a small part of the total cathode emission, there is no necessity to use oxide-coated cathodes in tubes constructed according to this invention.

The drawings are useful in explaining the invention. They have been chosen for the purpose of clear illustration of the principles involved and to illustrate conventionally a few only of the possible embodiments of the invention. In the drawings like numerals refer to like parts.

Figures 1 and 2 are diagrammatic sections of composite surfaces useful in explaining the invention.

Figure 3 is a diagrammatic section of a vacuum tube embodying the invention.

Figure 4 is a diagrammatic perspective view of an auxiliary cathode.

One of the most important aspects of the invention relates to the production in electron tubes of extremely high emission of secondary electrons as a consequence of strong electrostatic elds initiated by bombardment with primary electrons of composite surfaces constructed in accordance with the invention. There must, of course, be a suitable source of primary electrons and these electrons should have a speed such that the ratio of the secondary electrons released from the secondary-electron-emitting element to the primary electrons of the beam impinging on the secondaryelectron-emitting element is greater than unity.

The base, dielectric and secondary emitter are capable of being laid variously but always with the same resultant arrangement of layers and the same method of operation.

The metallic base of Figure 1 consists of a metal as later explained more in detail, which forms a self-limiting oxide dielectric layer 2| which is extremely thin as explained below. The metallic base 20 may also contain a secondaryelectron-emitter or a 'material which will form a secondary-electron-emitter as explained below.

The dielectric need not be a perfect insulator; it preferably will be a semi-conductor. The structure of the dielectric should preferably be very ne.

On the -dielectric layer is deposited a thin layer 22 of a substance which is a good emitter of secondary electrons such as alkali or alkaline earth metals, beryllium; beryllium oxide, BeO; beryl- 4 lium base alloys, particularly alloys of beryllium and copper; magnesium (oxidized) oxidized magnesium base alloys; aluminum base alloys of silicon, aluminum base alloys of magnesium, aluminum base alloys of copper, aluminum base alloys of beryllium, in which last alloys the content of beryllium is between 25 and 40 percent and in which aluminum base alloys of beryllium the combined content of magnesium, molybdenum and zirconium does not exceed two percent, oxides of aluminum base alloys having as a preconstituent beryllium, and aluminum base alloys of beryllium, having as a preconstituent magnesium in which last alloys the surface is oxidized. Typical examples of the aluminum base alloys are aluminum alloy 17S(Al 95%, Cu 4%, Mg 0.5%, Mn 0.5%) or aluminum alloy 24S (Al 93.8 Cu 4.2 Mg 1.5 Mn 0.5%) or the aluminum-magnesium (10% or 30 alloy when oxidized or the aluminum-beryllium alloys, of which the one containing 30% beryllium appears to be the most eiiicient. In the aluminum-magnesium alloys the surface should be oxidized. Among, the above, the duralumins are unexpectedly effective out of all proportion to any characteristics previously suspected and greatly exceed pure aluminum in multiplication.

When this top or secondary-electron-emitting layer 22 is struck by primary electrons under suitable potential conditions, it emits a large number of secondary electrons and thus becomes posi- -tively charged and creates a strong electrostatic field between the metallic base and the dielectric layer 2|. This electrostatic eld pulls out electrons from the metallic base 20 through the dielectric thus producing a high multiplication of electrons.

The combined thickness of the dielectric layer and of the layer emitting secondary electrons must be very small, the desirable range being from 2 to 20 microns. While not in every case essential, this range of thickness should be used for best results.

One of the problems in the prior art has been to cause the secondary-electron emission to stop and start either in coincidence with th-e `primary current or after only a brief and controllable time interval. Malter (Marconi) British Patent 481,170, September 7, 1936, uses caesium and is troubled by time lag between the start of the primary current and the start of the high secondary electron emission, as well as between the stopping of the primary current and the stopping of the secondary emission. (Malter, Physical Review, vol. 49, p. 478 and p. 879 (1936).) Furthermore, caesium deteriorates by volatilization in vacuo and causes objectionable photoelectric effects, which prohibit the use of the layer as an emitting surface in a photomultiplier tube.

In the present invention, the electron-emitting layer is preferably non-photoelectric, and many advantages and avoidance of much difficulty are thereby obtained.

In order to prevent excessive time lag, the positiv-e charge in the secondary-electron-emitting layer 22 must be neutralized lby electrons from the dielectric very quickly, but not quickly enough to interfere with extraction of secondary electrons by the electrostatic field. The extraction time has been estimated as about 10-14 seconds.

The dielectric layer 2l is formed on the metallic base by oxidation as later explained. The secondary-electron-emitting layer can be deposited on the dielectric by dusting, evaporation, settling, or the like. I have discovered that in order to produce the high field necessary for electron extraction from the metallic base under the conditions of cold emission (about 1,000,000 volts per centimeter) the thickness of the Idielectric layer is of importance. A vol-tage due to secondaryelectron-emission of more than a few thousand volts is not obtained in practice. For best results the thickness should be approximately 0.03 mm., and in any case the thickness of the layer should not exceed 0.1 mm. No limit on thinness is necessary provided the dielectric functions.

The invention is operative in its broader phases provided the metallic base, dielectric layer, and secondary-electron-emitting layer are as described, with-out further precautions to avoid time lag, but for best results special precautions to avoid time lag should be taken. There are several ways, which I have discovered to overcome this trouble.

Porosity of the dielectric Ipermits molecules of the secondary-electron-emitting layer at sufu ciently high temperatures to penetrate the dielectric, thus assisting in preventing time lag by conducting electrons to the surface and neutralizing the positive charge. Figure 2 shows at 22 projections from the secondary-electron-emitting layer into openings at 2l' in the dielectric, which will not act as a collector of pri-mary electrons but will form an interstitial impurity center for the dielectric.

Where the secondary-electron-emitter is beryllium, it has been successfully applied to the tube element by evaporation in vacuo from electrically heated tantalum spirals which serve as supports and heaters. It does not matter whether the beryllium is oxidized or not, since the metal and oxide are equally good as secondary emitters.

Where magnesium is used as a secondaryelectron-emitter, the oxidation is essential, as the unoxidized metal is not a good emitter. The metal will, however, oxidize rather readily in air. I have discovered that the leffectiveness of magnesium (oxidized) as a secondary-electronemitter is greatly improved by the step of baking in vacuo at a temperature of from 450" to 800 C., preferably 600 to 800 C. The reason for the improved results from the baking in vacuo is thought to be at least partly due to the migration of magnesium into the surface portion of the dielectric, and the fact has been clearly demonstrated.

The metallic base is made of a metal which forms an adherent oxide film self limited in thickness to about 50 Angstroms. The suitable metallic base materials may consist of aluminum or aluminum alloys predominantly consisting of aluminum, such as aluminum-copper -up to 11 aluminum-copper (4 to 42%)-manganese (0.5%)--magnesium (0.5 to 1.5%); aluminummagnesium aluminum-copper (4%)- silicon (3%) chromium, alloys predominantly consisting of chromium (such as chromiumcopper) copper, alloys predominantly consisting of copper (such as bronze, brass, Muntz metal), and corrosion-resisting iron-chromium (14%) alloys. A technique of preparing the 4composite surface is as follows:

The surface should be granulated into a mosaic. This can be done by firing in a hydrogen atmosphere at a temperature above 600 C. and below the softening point of the metal to reduce impurities and roughen the surface. It is then oxidized by exposure to an atmosphere of oxygen, avoiding contamination, as from oil on the fingers. If the surface is to be granulated, the surface is then etched, as by an acid or alkali in water. The oxidizing is important whether or not the etching is to be employed. The secondary-electron-emitting layer is then deposited on top. The granulation appears to assist by preventing too ready conductivity across the electron-emitting layer.

An alternate technique for producing the mosaic is to expose the composite surface (metallic base, oxide, and secondary-electron-emitting layer) abruptly and briefly to a temperature in the range from 600 C. to the softening point of the metallic base (preferably 600 to 800 C.) for aluminum and its alloys, and higher depending on the softening point for the other metals and alloys mentioned. In the case of a metal having a softening point higher than aluminum the temperature may range between 600 C. and the softening point of such metal. For best results the exposure should be limited to a few seconds in order to insure that the secondaryelectron-emitting layer (for example beryllium) be in the form of a well separated mosaic. The mosaic structure increases the surface of the dielectric exposed to the primary beam and reduces conductivity in the plane parallel to the metallic base.

Suitable secondary-electron-emitters have already been enumerated. The composite surface (metallic base, oxide, and secondary-electronemitter) may be prepared as follows: lIhe metal base is nred at a temperature above 500 C. and below the softening point of the metal in an atmosphere of hydrogen and then in vacuo. It is then oxidized by exposure to an atmosphere of oxygen. The layer of secondary-electron-emitter is then deposited on top by one of the methods already enumerated, such as dusting, evaporation, settling or the like. The precise method used will preferably vary with the nature of the material selected for the metallic base and the secondaryelectron-emitter.

The following is an example of a typical procedure, which, however, does not limit to the detail thereof The secondary electron emitting element has a tantalum base, which is supported in a glass envelope. The assembly is baked out at 450 C. with an end vacuum of better than 10-6 mm. of mercury. While maintaining this vacuum, the tantalum is heated individually to about 2000 C. for several minutes to remove impurities. Next, potassium chloride is evaporated on the metallic base, tantalum, by heating inductively a metal directional evaporator containing potassium chloride to a temperature of about 1200 C. for about 15 minutes. A layer of magnesium a few microns thick is evaporated on this dielectric from a tungsten wire evaporator with directional shields. During the two evaporations the temperature of the metallic base is approximately at room temperature, and the vacuum is maintained near its initial value of 106 mm. of mercury or better. The element and glass envelope are then baked for about one hour at 450 C. for the magnesium to penetrate the potassium chloride in a good vacuum. The processing is then nished.

There is no exact temperature at which beading or mosaic formation occurs, but rather a range as set forth within which the phenomenon is evidenced. The extent of mosaic formation is influenced by such factors as oxide film thickness and heat capacity ofthe metallic base. The degree to which the .metal base must be purified depends on the technique by which the metal composing the base has been prepared.

On the other hand a very satisfactory secondary-electron-emitter is produced by an alloy predominantly consisting of a metal which forms an adherent oxide, such as aluminum, copper, or chromium, or corrosion resisting chromium iron alloys, self limited to 50 Angstroms thickness, and containing a substantial amount and up to 30% (preferably up to 10%) of a metal such as beryllium and/or magnesium which is an excellent secondary-electron-emitter, either as metal or oxide or both.

It will usually not be profitable to employ less than 1% of the preferred secondary-electronemitting element in the alloy.

The alloy must be substantially free from poisoning ingredients such as nickel or, less importantly, cobalt. Aluminum has been found to give best results for the predominant metal of the alloy.

The alloy should be treated to concentrate the preferred secondary-electron-emitting component at the surface by metal migration. This can be accomplished by firing for a few seconds in hydrogen at 600 C. to 800 C. or by burnishing under an inert organic liquid such as benzene, toluene, or xylene. In either case this should precede oxidizing.

The method of bringing the preferred secondary-electron-emitter to the surface as described in Junker and Leitgebel U. S. Patent No. 2,254,805 may also be used. The alloy is then oxidized as by exposure to the air at ordinary temperature. A preferred method of oxidation in alloys predominantly consisting of aluminum is to immerse in an alkaline solution of an oxidizing agent, such as ferric hydrate, heated to about 115 C., for a few minutes, depending on the desired oxide thickness. In this way the resultant is first the alloy forming a base metal layer, then a mixture of oxides containing particles of the dominant metal and its oxide, and finally a secondary-electron-emitting layer consisting chiefly of BeO, MgO, or a mixture. The mixture of oxides containing particles of the dominant metal and its oxide is a P or N type semi-conductor depending on whether oxygen or metal is in excess from stoichiometric proportions. The same is true of the adherent oxide layer on the metal base when the secondary-electron-emitting layer is separately applied and is not formed from an ingredient in the alloy of the base metal. The electron emitting properties of this combination are very high.

The composite surfaces just described may be utilized in several distinct types of apparatus, as Will be evident from the following discussion.

For electron photo-multipliers such composite surfaces will be useful particularly for cathodes emitting-secondary-electrons under electron bombardment. As Well known, metals Whose surfaces are covered with alkali or alkaline earth metals have high emissive properties (Gorlich U. S. Patent No. 2,317,754). By the features of the present invention set forth herein, the secondary emission is still further enhanced. As well known in the art, such electron photo-multipliers may serve various functions, some of which are shown by Slepian U. S. Patent No. 1,450,265 or Piore U. S. Patent No. 2,123,024.

Proper adjustment of the conductivity of the dielectric in the composite surface under the primary electron beam is contemplated.

Secondary-electron-emitters according to the invention may also be used to increase the efficiency of thermionic vacuum tubes or gaseous tubes such as diodes, triodes, thyratrons, and the like. The thermionic cathodes in present use require a relatively large electrical input in order to produce any appreciable current from cathode to anode. In accordance with this aspect of the invention, a large electron current Ican be produced by an auxiliary cathode constructed in accordance with any one of the methods for producing composite surfaces with enhanced secondary-electron-emission when this auxiliary cathode is bombarded by a weak primary electron current. Only a small power input is required to produce this weak primary electron current by thermionic emission.

An equally small additional power input is necessary to supply the voltage to the thermionic electrons to give them the energy required to produce very high secondary-electron-emission when they strike any one of the multiplier elements earlier described. rfhe auxiliary cathode can be used in diodesy triodes, many grid tubes, thyratrons, and the like.

Figure 3 shows this device applied in a triode having an envelope 23 suitably evacuated and containing an anode 2li, a grid 25, a cathode 26, and an auxiliary cathode 2 in the form of a mesh.

In electron multipliers, mesh secondary-electron-emitters have been developed by G. Weiss, U. S. Patent No. 2,243,178 dated May 27, 1941. Figure 3 is a diagrammatic view of the arrangement of tube elements in a triode with cylindrical electrodes. Figure 4 gives a schematic view of the cathode and auxiliary cathode. It will be evident that the auxiliary cathode 27 is supported at 23, and the cathode 2t is supported at 38. Other elements of the tube may take on any of their well known forms,

It will be evident that secondary-electronemitters with high secondary-emission in combination with dielectrics and metallic bases may be used to advantage in other ways well known in the art.

The auxiliary cathode may be used to overcome certain disadvantages associated with the use of thermionic cathodes, such as backring, grid emission and ignition of local spots. In standard tubes with oxide-coated cathodes, some of the cathode material, such as barium, evaporates and settles on the grid. rIhis contaminates the grid and gives rise to grid emission of electrons, disrupting the normal functioning of the tube.

Backring is conduction in the reverse direction (plate to cathode) when the plate potential is negative with respect to cathode. It is especially harmful in rectifiers. Backring is due to a deposit of electron-emitting substance on the plate. This cannot occur in the present invention since the secondary-electron-emitter may be chosen so as not to evaporate at the operating temperature of the tube. However, in a very high frequency oscillator the plate may have a coating of secondary-electron-emission material to enhance the efficiency of the tube.

Local hot spots are due to uneven heating of the oxide coated cathode. They cause increased local evaporation and deterioration of the cathode. The cathode material used in my invention will not evaporate at the operating temperature chosen for the tube, thus eliminating both diiculties.

The prior art, in seeking to overcome the difficulties, produced remedies not generally applicable. See Ct. Jobst, U. S. Patent 1,964,517,dated June 26, 1934; W. Espe, U. S. Patent 2,125,105, dated July 26, 1938; and H. Kolligs, U. S. Patent No. 2,147,447, dated February 14, 1939. These prior art devices do not make use of auxiliary cathodes coated with suitable emitters of secondary-electrons to eliminate all these dii'liculties.

As a result of investigation,- the necessary features for a secondary-electron-emitter which will function efficiently as an auxiliary cathode have been determined. The following remarks apply particularly to eliminating grid contamination in high power triodes, but they apply equally `well to eliminating this and other difficulties in various types of electron tubes. Oxide coated cathodes, now in vogue in high powered triodes, are used because they require the least energy input per unit thermionic output at saturation. These oxide coated cathodes cause .grid contamination.

In accordance with the present invention, the primary thermionic current need be only a small fraction of the total cathode emission, the main part being supplied by secondary electron-s from the auxiliary cathode.

Hence, the oxide coated cathode need not be used, and instead the cathode of the invention will desirably be uncoated, made of tungsten, thoriated tungsten or other material which will not evaporate at the operating temperature of the tube. The auxiliary cathode will have an outer layer Of beryllium or other material as set forth above, which will have high secondary electron emission, but will not be effected either by melting or evaporating, at the operating temperature of the tube. To prepare the mesh described in Figures 3 and 4, the metallic base of the secondary cathode will best be aluminum or equivalent as explained above.

In view of my invention and disclosure variations and modifications to meet individual whim or particular need will doubtless become evident to others skilled in the art, to obtain all or part of the benets of my invention without copying the process shown, and I, therefore, claim all such insofar as they fall within the reasonable spirit and scope of my claims.

Having thus described my invention what I claim as new and desire to secure by Letters Patent is:

1. The process of increasing the emission of secondary-electrons from a multiplier element including a metallic base in an electron tube, which comprises forming the metallic base predominantly from a metal selected from the class consisting of aluminum, copper, chromium and the corrosion resisting iron-chromium alloys having as a minor constituent up to 30 per cent of an electron emitter metal of the class consisting of beryllium and magnesium and substantially free from nickel and cobalt, concentrating the electron-emitter at the surface by metal migration and as a separate step oxidizing the multiplier element.

2. rl'he process of increasing the emission of secondary-electrons from a multiplier element including a metallic base in an electron tube, which comprises forming the metallic base predominantly from a metal selected from the class consisting of aluminum, copper, chromium and corrosion resisting iron-chromium alloys, having as a minor constituent a metal selected from the class consisting of beryllium and magnesium but substantially free from nickel and cobalt, forming the electron emitting surface by ring in 10 hydrogen at 600 to 800 C. for a short time and then as a separate step oxidizing the surface.

3. The process of increasing the emission of secondary-electrons from a multiplier element including a metallic base in an electron tube, which comprises forming the metallic base predominantly from a metal selected from the class ccnsisting of aluminum, copper, chromium and the corrosion resisting iron-chromium alloys having as a minor constituent up to 30 percent of an electronemitter metal of the class consisting of beryllium and magnesium and substantially free from nickel and cobalt, concentrating the electron-emitter at the surface by metal migration, as a separate step oxidizing the multiplier element, and granulating the surface into a mosaic.

4. The process of increasing the emission of secondary-electrons from a multiplier element including a metallic base in an electron tube, which comprises forming the metallic base predominantly from a metal selected from the class consisting of aluminum, copper, chromium and corrosion resisting iron-chromium alloys, having as a minor constituent a metal selected from the class consisting of beryllium and magnesium but substantially free from nickel and cobalt, forming the electron emitting surface by firing in hydrogen at 600 to 800 C. for a short time, then as a separate step oxidizing the surface, and etching the surface.

5. The process of increasing the emission of secondary-electrons from a multiplier element including a metallic base in an electron tube, which comprises forming the metallic base predominantly from a metal selected from the class consisting of aluminum, copper, chromium and the corrosion resisting iron-chromium alloys having as a minor constituent up to 30 percent of an electron emitter metal of the class consisting of beryllium and magnesium and substantially free from nickel and cobalt concentrating the electron-emitter at the surface by metal migration, and subsequently exposing the metallic base to oxygen for a duration suflicient to oxidize the surface.

6. The process of increasing the emission of secondary-electrons from a multiplier element including a metallic base in an electron tube, which comprises forming the metallic base predominantly from a metal selected from the class consisting of aluminum, copper, chromium and corrosion resisting iron-chromium alloys, having as a minor constituent a metal selected from the class consisting of beryllium and magnesium but substantially free from nickel and cobalt, forming the electron emitting surface by firing in hydrogen at a temperature between 600 C. and the softening point of the metallic base for a short time and then as a separate step oxidizing the surface.

7. In the process of increasing the emission of secondary-electrons from a multiplier element, including a metallic base in an electron tube, the steps which comprise forming the metallic base predominantly from a metal selected from the class consisting of aluminum, copper, chromium and` corrosion resisting iron-chromium alloys, having as a minor constituent a metal selected from the class consisting of beryllium and magnesium but substantially free from nickel and cobalt, forming the electron emitting surface by firing in hydrogen at a temperature between 500 C. and the softening point of the metallic base and subsequently exposing the metallic base to 11 12 oxygen for a duration sufcent to oxdize the Number Name Date surface. 1,981,620 Gard Nov. 20, 1934 JENNY BRAMLEY. 1,985,855 Edwards et a1. Dec. 25, 1934 2,075,377 Varian Mar. 30, 1937 REFERENCES CITED 5 2,390,701 Ferris Dec. 11, 1945 The following references are of record in the 2,531,382 Arditi N0V- 28, 1950 111e Of this patent: FOREIGN PATENTS UNITED STATES PATENTS Number Country Date Number Name Date 10 862,488 France Dec. 9, 1940 1,694,189 Ruben Dec. 4, 1928 862,489 France Dec. 9, 1940 1,720,675 Hertz l- July 16, 1929 862,490 France Dec, 9, 1940 1,926,407 Ruben Sept. 12, 1933 

1. THE PROCESS OF INCREASING THE EMISSION OF SECONDARY-ELECTRONS FROM A MULTIPLIER ELEMENT INCLUDING A METALLIC BASE IN AN ELECTRON TUBE, WHICH COMPRISES FORMING THE METALLIC BASE PREDOMINANTLY FROM A MEAL SELECTED FROM THE CLASS CONSISTING OF ALUMINUM, COPPER, CHROMIUM AND THE CORROSION RESISTING IRON-CHROMIUM ALLOYS HAVING AS A MINOR CONSTITUENT UP TO 30 PER CENT OF AN ELECTRON EMITTER METAL OF THE CLASS CONSISTING OF BERYLLIUM AND MAGNESIUM AND SUBSTANTALLY FREE FROM NICKEL AND COBALT, CONCENTRATING THE ELECTRON-EMITTER AT THE SURFACE BY METAL MIGRATION AND AS A SEPARATE STEP OXIDIZING THE MULTIPLIER ELEMENT. 